The present disclosure relates to a step-down switching regulator which supplies a power source voltage to various electronic apparatuses.
The step-down switching regulator is used in, for example, a mobile electronic apparatus to step down an input battery voltage to a predetermined voltage and supply the predetermined voltage to an electronic circuit.
Referring to FIG. 9, such a step-down switching regulator includes an input terminal 1a through which an input voltage Vi is applied from an input power source 1, such as a battery or the like, a switching device 2 and a rectifier switching device 3 which are connected in series between the input terminal 1a and a common potential and are alternately switched ON and OFF, an inductor 4 having one end connected to a connection point between the switching device 2 and the rectifier switching device 3, an output capacitor 5 which smoothes a voltage at the other end of the inductor 4 to generate an output voltage Vo at an output terminal 1b, and a control circuit 60 which detects the output voltage Vo and drives the switching device 2 and the rectifier switching device 3 so that the output voltage Vo becomes equal to a set target voltage.
In the aforementioned configuration, when the switching device 2 is ON and the rectifier switching device 3 is OFF, a current flows from the input terminal 1a via the switching device 2 and the inductor 4 to the output terminal 1b. In this case, a difference voltage between the input voltage Vi and the output voltage Vo is applied to the inductor 4, which is therefore energized. Next, when the switching device 2 is OFF and the rectifier switching device 3 is ON, a current flows via the rectifier switching device 3 and the inductor 4 to the output terminal 1b. In this case, the output voltage Vo is applied to the inductor 4, which is therefore de-energized. In a steady state, the amounts of energization and de-energization of the inductor 4 are balanced, i.e., the product of an applied voltage and a period of time is the same between during energization and during de-energization, and therefore, the following relationship is satisfied:(Vi−Vo)×Ton=Vo×Toff  (1)where Ton represents an ON time period of the switching device 2 and Toff represents an OFF time period of the switching device 2. According to Expression (1), the output voltage Vo is represented by:Vo=Vi×Ton/(Ton+Toff)  (2)
The output voltage Vo is adjusted by the proportion (D=Ton/T, referred to as a duty ratio D) of the ON time period in a cycle T (=Ton+Toff) of the switching device 2.
As can be seen from Expression (2), if the input voltage Vi decreases as the battery (i.e., the input power source 1) is consumed, the duty ratio D of the switching device 2 increases so as to stabilize the output voltage Vo. For example, if duty ratio D is close to 1 (e.g., D=0.98), the OFF time period of the switching device 2 is excessively short, so that the circuit actually has difficulty in performing stable operation. Specifically, the switching device 2 repeats irregular switching between the ON state (D=1) and the switching operation, resulting in variations in the output voltage Vo.
As a technique for solving this problem, for example, Japanese Unexamined Patent Application Publication No. S60-257766 discloses a step-down switching regulator which switches the switching device 2 from switching regulator operation to series regulator operation when the input voltage Vi decreases to less than a predetermined value.
Also, U.S. Pat. No. 6,472,854 discloses a step-down switching regulator, as shown in FIG. 10, which includes a bypass transistor 7 between an input and an output thereof, and activates the bypass transistor 7 when detecting that the duty ratio D of the switching device 2 is 1.
In FIG. 10, a drive circuit 22 which drives the bypass transistor 7 includes an N-channel FET 221, a resistor 222, a capacitor 223, and a diode 224. The capacitor 223 is charged via the resistor 222 when the switching device 2 is ON, and is discharged via the diode 224 and the rectifier switching device 3 when the switching device 2 is switched OFF and the rectifier switching device 3 is switched ON. The charging time constant of the capacitor 223 is set so that the N-channel FET 221 is not switched ON during normal switching operation of the switching device 2. Therefore, the bypass transistor 7 is maintained in the OFF state.
When the input voltage Vi decreases close to the output voltage Vo and the duty ratio D of the switching device 2 reaches 1, the capacitor 223 is no longer discharged, and therefore, the gate voltage of the N-channel FET 221 is pulled up by the resistor 222, so that the N-channel FET 221 is caused to be in the ON state. As a result, the bypass transistor 7 is also caused to be in the ON state, and therefore, operation where the duty ratio D=1 is established, and in addition, the ON resistance of the bypass transistor 7 is connected in parallel to a serial resistance of the ON-resistance of the switching device 2 and the inductor 4, so that a capability to supply a current to the output terminal 1b is improved.